Adsorption and Reaction of Methyl and Ethyl Iodide on Potassium-Promoted Mo₂C/Mo(100) Surface

Abstract

X-ray photoelectron spectroscopy (XPS) studies revealed that potassium on Mo 2 C/Mo(100) induced cleavage of the C-I bond in adsorbed CH 3 I even at ∼100 K. The temperature of complete C-I bond breaking occurred 60-80 K lower compared to the clean surface. Preadsorbed potassium also influenced the reaction pathway of adsorbed CH 3 formed. It decreased its self-hydrogenation into methane and facilitated the coupling reactions into ethane and ethylene below 200 K. High-resolution electron energy loss spectroscopy (HREELS) confirmed formation of adsorbed CH 3 species and revealed its thermal stability. Potassium exerted a similar influence on the chemistry of C 2 H 5 I on Mo 2 C/Mo(100) surface. Rupture of the C-I bond also occurred more easily on K-covered surface. Ethyl species, the primary product of the dissociation, dehydrogenated into ethylene on one hand and hydrogenated into ethane on the other. Rupture of the C-C bond was not observed even at high potassium coverage. Illumination of coadsorbed layers promoted further dissociation of the C-I bond in both compounds at ∼100 K.

Density Functional Theory Study of Methyl Iodide Adsorption and Dissociation on Clean and K-Promoted β-Mo2C Surfaces

“…45 In Figure 4 the work function changes of the above surfaces are plotted as a function of CH 3 I exposure performed at 100 K. On both surfaces, a nearly linear change in WF was found up to ∼4 L exposure. It was observed previously 15 that 0.35 ML K promoted the decomposition of CH 3 I at 115 K only about to 5% following 4 L exposure. Accordingly, the WF values measured as a result of CH 3 I uptake (Figure 4) reflect the charge transfer to the intact molecule up to 4 L exposure, except those measured at the lowest exposures.…”

“…Although similar adsorption energies were obtained at different adsorption sites, a preferential tricoordinated model at low coverage was obtained on similar systems. 34,35 According to the experimental procedure in ref 15, K was added neutral prior to CH 3 I adsorption. For this system, surface Mo and subsurface C layers, and also the K atom, were allowed to fully relax during optimization.…”

“…The above argument involves similar surface concentration for methyl iodide at a given exposure on the K-free and K-dosed surfaces, which was really observed by XPS measurements. 15 Table 4 shows the computed overlap population, for bonds on isolated and adsorbed methyl iodide. We can see in all cases that the OP value for the C-I bond reduces when methyl iodide is adsorbed, resulting in the weakening of the C-I bond before adsorption.…”

“…We also studied the primary dissociation process of an adsorbed CH 3 I molecule yielding adsorbed CH 3 and I (CH 3 I = CH 3(a) þ I (a) ). The formation of CH 3 fragments has been detected by HREEL spectroscopy on the K-dosed Mo 2 C surface even at 100 K. 15 We investigated the dissociation process and the effect of K by first analyzing the stability of adsorbed species on the surface and then modeling a plausible reaction path. The preferential adsorption sites for the CH 3 on a clean and K-promoted Mo 2 C surface were found by placing the fragment at different positions on the surface and performing a full geometry optimization.…”

“…For this reason, we have addressed the effect of an alkali metal, potassium, on the reaction path of the simple alkyl group, CH 3 , on the Mo 2 C surface, which behaves similarly to platinum metals in many respects . The effect of potassium on the reaction path of the methyl iodide molecule on molybdenum carbide was that the surface concentration of methyl groups could be remarkably enhanced, and their decomposition to surface carbon was suppressed, allowing higher conversion toward coupling products . Note that on molybdenum carbide the reactivity of other alkyl iodides − toward adsorbed potassium was similar to that of methyl iodide, and potassium was also reactive with other alkyl halides on other surfaces as well, for example, with CH 3 Cl on Pd(100) and Rh(111) crystal planes.…”